Tuesday, December 25, 2007

God and his attributes

God most normally refers to the holy being worshipped by supporters of monotheistic and monolatrist religions, whom they accept as true to be the creator and ruler of the world.

Theologians have ascribed different attributes to a variety of conceptions of God. The most usual among these consist of omniscience, perfect goodness, omnipotence, omnipresence, divine simplicity, and everlasting and necessary existence. God has also been conceived as being incorporeal, a personal being, the foundation of the entire moral obligations, and the "greatest conceivable existent". These attributes were all supported to unreliable degrees by the early Christian, Jewish and Muslim theologian philosophers, including Augustine of Hippo, Al-Ghazali, and Maimonides. Many famous medieval philosophers developed arguments for the existence of God, attempting to fight with the obvious contradictions implied by many of these attributes.

Monday, December 17, 2007

Bridges in 18th century

Rope bridges, a simple type of postponement bridge, were used by the Inca civilization in the Andes mountains of South America, just prior to European colonization in the 1500s.

During the 18th century there were many innovations in the plan of timber bridges by Hans Ulrich, Johannes Grubenmann, and others. The earliest engineering book on building bridges was written by Hubert Gautier in 1716.

With the Industrial Revolution in the 19th century, truss systems of wrought iron were urbanized for larger bridges, but iron did not have the tensile strength to support large loads. With the advent of steel, which has a high tensile power, much larger bridges were built, many using the ideas of Gustave Eiffel.

Wednesday, December 12, 2007

History of bridges

The first bridges were spans complete of wooden logs or planks and eventually stones, using a simple support and crossbeam arrangement. Most of these early bridges were extremely poorly built and could rarely support heavy weights. It was this inadequacy which led to the growth of better bridges. The arch was first used by the Roman Empire for bridges and aqueducts, some of which still place today. These arch based bridges could place in conditions that would previously have swept any bridge away.

An example is the Alcantara Bridge, built in excess of the river Tagus. Most former bridges would have been swept away by the strong current. The Romans also used cement, which condensed the variation of strength found in natural stone. One kind of cement, called pozzolana, consisted of water, lime, sand, and volcanic rock. Brick and mortar bridges were built after the Roman era, as the technology for cement was lost then afterward rediscovered.

Tuesday, December 04, 2007

Grape

A grape is the non-climacteric fruit that grows on the permanent and deciduous woody vines of the folks Vitaceae. Grapes develop up in clusters of 6 to 300, and can be black, blue, golden, green, purple, red, pink, brown, peach or white. They can be eaten raw or used for producting jam, grape juice, jelly, wine and grape germ oil. Development of grapevines occurs in vineyards, and is called viticulture. One who studies and practises increasing grapes for wine is called a viticulturist. The leaves of the grape vine itself are painstaking safe to eat and are used in the production of dolmades.

Monday, November 26, 2007

Salad

Salad is a light meal — or, as part of a bigger meal, much more of an taster — consisting of varied vegetables (usually including at least one leaf vegetable) or fruit, frequently with a dressing or sauce, occasionally nuts and sometimes with the addition of meat, fish or cheese. It is usually seen as a healthy dish, even though not always low in calories, salt, sugar, or fat because of the dressing that is often added. The word "salad" comes from the French salade of the identical meaning, which in twist is from the Latin salata, "salty", from sal, "salt".

Monday, November 19, 2007

Cookie

In the United States and Canada, a cookie is a tiny, round, flat cake. In most English-speaking countries outer North America, the most common word for this is biscuit; in many regions both terms are used, while in others the two words have diverse meanings—a cookie is a bun in Scotland, while in the United States a biscuit is a kind of quick bread not unlike a scone.

Cookies can be baked until crisp or just long enough that they stay soft. Depending on the kind of cookie, some cookies are not cooked at all. Cookies are made in a broad variety of styles, using an array of ingredients including sugars, spices, chocolate, butter, peanut butter, nuts or dried fruits. The smoothness of the cookie may depend on how long it is baked.

Monday, November 12, 2007

Present Battery Electric Vehicles

At the present time, disagreement reign over battery electric vehicles. Campaigners, (et al) for BEV's are reproving three major US automobile manufacturers of deliberately sabotaging BEV efforts through more than a few methods, for instance, failing to market, failing to produce appropriate vehicles, by failing to satisfy demand and using lease-only programs with prohibitions against end of lease purchase.

In their defense, the three major manufacturers they have responded that they only create what the public want and the recent trend is that the public doesn't want battery electric vehicles.Although we have the technology to manufacture and provide BEVs, one of the largest downfalls for the prolific production of BEVs is the extortionate cost of replacement batteries. In some cases the cost of stand-in batteries can be more than the price of the whole vehicle, especially when buying used battery electric vehicles.

Monday, November 05, 2007

Electric vehicles in 1959

In 1959 the Henney Kilowatt was introduced and was the world's initial modern transistor-regulated electric car and the predecessor to the more recent battery electric vehicles such as General Motors EV1. Only 47 Henney Kilowatts were created, 24 being sold as 1959 models and 8 as 1960 models. It is not clear what happened to the other 15 built but it could be probable that they were sold as 1961 or 1962 models. None of the 8 1960 models were sold in the direction of the public because of the high manufacturing costs, but were sold to the electric cooperatives who funded the project.

It is estimated that there are connecting four and eight Henney Kilowatt battery electric vehicles still in existence with at least two of the survivors still driven at times.

Battery electric vehicles had issues with high battery costs, with restricted travel distances, with charging time and the lifespan of the battery, although advancements in battery technology has addressed a lot of those problems.

Monday, October 29, 2007

Battery in1913 to 1947

One of the downfalls of the battery electric vehicle was the opening of the electric starter in 1913. It easy the task of starting an internal combustion engine which was previously hard and dangerous to start with the crank handle. Another was the mass-produced and somewhat cheap Ford Model-T. Finally, the loss of Edisons through present electric power transmission system. He was battling with George Westinghouse and Nikola Tesla over their desire to introduce alternating present as the principal electricity distribution. Edison's direct current was the weight for electric motors.

Battery electric vehicles were limited to position applications. Forklift trucks were battery exciting vehicles when introduced in 1923. BEV golf carts which were used as locality electric vehicles and were partially "street legal". By the late 1930s, the electric automobile business had disappeared until the invention of the point contact transistor in 1947 which started a new era of electric vehicle.

Monday, October 22, 2007

History of Battery Electric Vehicles in 1900

Prior to 1900, battery electric vehicles held many speed and distance records, the most famous of which, was the breaking of the 100 km/h (60 mph) speed barrier. It was by Camille Jenatzy on April 29, 1899 in a rocket-shaped vehicle named Jamais Contente (Never Happy) which reached a peak speed of 105.88 km/h (65.79 mph).

During the early 20th Century, battery electric vehicles outsold gasoline powered vehicles and were successfully sold as city cars to upper-class customers. Because of high technological limitations, these cars were limited to a peak speed of about 32 km/h (20 mph). The cars were marketed as "appropriate vehicles for women drivers". Electric vehicles did not need hand-cranking to launch.

Monday, October 15, 2007

The History of Battery Electric Vehicles

Battery exciting Vehicles or BEVs, predated the Internal Combustion Engine (ICE) vehicles. It was between 1832-1839 that Robert Anderson, a Scottish businessman, imaginary the first electric carriage and Professor Sibrandus Stratingh from the Netherlands designed the first small-scale electric car which was built by his assistant Christopher Becker in 1835.

The storage battery better, firstly by Gaston Plant, a French physicist who invented the guide acid cell in 1859 and the first rechargeable battery. Then, in 1881, Camille Faure residential a more efficient and reliable battery which became so successful in the early electric cars. This discovery caused battery electric vehicles to flourish, with France and Great Britain being the first nations to support prevalent development of electric vehicles.

Monday, October 08, 2007

Battery


A battery is a device consisting of one or more electrochemical cells, which store chemical energy and make it accessible in an electrical form. There are many types of electrochemical cells, together with galvanic cells, electrolytic cells, fuel cells, flow cells, and voltaic cells. Formally, an electrical "battery" is an array of similar voltaic cells ("cells") joined in series. However, in many contexts it is universal to call a single cell a battery. A battery's individuality may vary due to many factors including internal chemistry, current drain, and temperature. Generally, battery life can be prolonged by storing the battery in a cool place and using it at an appropriate current.

Although an early form of battery may have been used in ancient times, the development of modern batteries started with the Voltaic pile, invented by the Italian physicist Alessandro Volta in 1800. Since then, batteries have gained recognition as they became portable and useful for many purposes. Unfortunately, the well-known use of batteries has created many environmental concerns, such as toxic metal pollution. Many reclamation companies reprocess batteries to reduce the number of batteries going into landfills. Rechargeable batteries can be charged hundreds of times before draining out; and even after wearing out they can be recycled.

There are two types of batteries disposable and rechargeable both of which convert chemical energy to electrical energy. Disposable batteries can only be used once because they use up their chemicals in an irreversible reaction. Rechargeable batteries can be recharged because the chemical reactions they use are reversible; they are recharged by running a charging current through the battery, but in the opposite direction of the discharge current.

Monday, September 24, 2007

Metal

In chemistry, a metal is a ingredient that readily loses electrons to form positive ions and has metallic bonds between metal atoms. Metals form ionic bonds with non-metals. They are sometimes described as a web of positive ions surrounded by a cloud of delocalized electrons. The metals are one of the three groups of elements as eminent by their ionization and bonding properties, along with the metalloids and nonmetals. On the periodic table, a diagonal line drawn from boron separates the metals from the nonmetals. Most elements on this line are metalloids, sometimes called semi-metals; elements to the lower left are metals; elements to the upper right are nonmetals.

Monday, September 17, 2007

Pollination

Pollination is an important step in the reproduction of seed plants: the transfer of pollen grains (male gametes) to the plant carpel, the organization that contains the ovule (female gamete). The accessible part of the carpel is called a stigma in the flowers of angiosperms and a micro Pyle in gymnosperms. The study of pollination brings mutually many disciplines, such as botany, horticulture, entomology, and ecology. Pollination is significant in horticulture because most plant fruits will not develop if the ovules are not fertilized. The pollination process as communication between flower and vector was first addressed in the 18th century by Christian Konrad Sprengel.

Monday, September 10, 2007

Troposphere

From the Greek word "τρέπω" importance to turn or mix. The troposphere is the lowest layer of the atmosphere; it starts at the surface and extends to between 7 km (23,000 ft) at the poles and 17 km (60,000 ft) at the equator, with some distinction due to weather factors. The troposphere has a enormous deal of vertical mixing due to solar heating at the surface. This heating warms air masses, which makes them less intense so they rise. When an air mass raises the force upon it decreases so it expands, doing work against the contrasting pressure of the surrounding air. To do work is to use energy, so the temperature of the air mass decreases. As the temperature decreases, water vapor in the air mass may concentrate or solidify, releasing latent heat that further uplifts the air mass. This process determines the maximum rate of refuse of temperature with height, called the adiabatic lapse rate.

Tuesday, September 04, 2007

water taxi

A water taxi or river taxi or aquatically disposed taxi is a boat used for public transportation in cities with plentiful water channels. Many cities, including New York City, Boston, Baltimore, Fort Lauderdale, Winnipeg, Vancouver, London, and Tokyo have planned water taxis that operate in a similar manner to ferries or buses. Others, like Venice, have for-hire boats like to traditional taxis. Venice also has a vaporetto or waterbus system that operates in the same way to American "water taxis" (image).
Water taxis also activate in cottage areas where some cottages are available only by water. Visitors can drive to a local marina and take a water taxi to the final purpose.
On March 6, 2004, a "Seaport Taxi," a water taxi service operated by the Living Classrooms Foundation, capsized through a storm near Baltimore's Inner Harbor; 5 passengers died in the accident.

Monday, August 27, 2007

Leaf vegetable


Leaf vegetables, also called potherbs, greens, or leafy greens, are plant leaves eat as a vegetable; sometimes attend by tender petioles and shoots. Although they come from a very broad diversity of plants, most share a great deal with other leaf vegetables in nutrition and cooking methods.

Nearly one thousand types of plants with edible leaves are known Leaf vegetables most often come from short-lived herbaceous plants such as lettuce and spinach. Woody plants whose leaves can be eaten as leaf vegetables include Adenosine, Aralia, and Moringa, Morus, and Toona species.

The leaves of many fodder crops are also edible by humans, but frequently only eaten under famine conditions. Examples include alfalfa, clover, and most grasses, as well as wheat and barley. These plants are often much more prolific than more traditional leaf vegetables, but utilization of their rich nutrition is difficult, primarily because of their high fiber content. This obstacle can be overcome by further giving out such as drying and grinding into powder or pulping and pressing for juice.

During the first half of the 20th century many grocery stores with vegetable sections sold small bunch of herbs tied with a thread to small green and red peppers known as "potherbs."

Tuesday, August 21, 2007

Earth

Earth often known as the Earth, Terra, the World or Planet Earth is the third planet away from the Sun, and is the fifth largest planet in the solar system. It is also the largest of its terrestrial system's global planets, making it the largest solid body in the solar system, and it is the only position in the universe known to humans to support life. It as well the densest planet in the lunar system. The Earth was shaped around 4.57 billion years ago and its natural satellite, the Moon, was orbiting it shortly thereafter, around 4.53 billion years ago.

The outer surface is divided into several tectonic plates that gradually travel across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convicting yet solid layer and an iron core that generates a magnetic field. Its atmospheric conditions have been considerably altered by the presence of life forms, which make an ecological balance that modifies the surface situation. About 71% of the surface is covered in salt-water oceans, and the remaining consists of continents and islands.

Monday, August 13, 2007

Chef's uniform

The conventional chef's uniform, including toque (traditional hat), white double breasted jacket, and checked pants are immediately recognized by most members of the Western world, especially in this day of television's celebrity chefs. The double breasted jacket can be inverted to conceal stains. Its thick cotton cloth protects from the heat of stove and oven and protects from splattering of steaming liquids. An apron is an obviously useful piece of utensils used to guard the rest of the wearer's garments from food splatters and stains.

The toque (chef's hat) dates back to the 16th century when hats were regular in many businesses. Different heights of hats point out rank within a kitchen. Some modern chefs have put their own diverse whirl on the traditional uniform. But the traditional, practical, clothing of the chef still remainders a standard in the food industry.

Tuesday, August 07, 2007

Infrared

Infrared (IR) emission is electromagnetic emission of a wavelength longer than that of noticeable light, but shorter than that of radio waves. The name means "below red", red being the color of detectable light of longest wavelength. Infrared radiation spans three instructions of magnitude and has wavelengths between about 750 nm and 1 mm.

These divisions are suitable by the different human response to this radiation: near infrared is the area closest in wavelength to the radiation detectable by the human eye, mid and far infrared are gradually further from the visible regime. Other definitions follow different physical mechanisms and the newest follow technical reasons .Unfortunately the international standards for these specifications are not currently obtainable.

The boundary between visible and infrared light is not precisely defined. The human eye is markedly less responsive to light above 700 nm wavelength, so longer frequencies make irrelevant contributions to scenes illuminated by common light sources. But particularly strong light (e.g., from lasers, or from bright daylight with the visible light removed by colored gels can be detected up to approximately 780 nm, and will be apparent as red light. The onset of infrared is defined at different values typically between 700 nm and 780 nm.

Monday, July 30, 2007

Software

Software basically is the distinct image or representation of physical or material position that constitute configuration to or functional identity of a machine, usually a computer. As a substance of memory, software in principle can be changed without the alteration to the static paradigm of the hardware thus without the remanufacturing thereof. Generally software is of an algorithmic form which translates into being to a progression of machine instructions. Some software, however, is relational forms which translate into being the map of a recognition network.

Software is a program that enables a computer to achieve a specific task, as contrasting to the physical components of the system. This include application software such as a word processor, which enables a user to achieve a task, and system software such as an operating system, which enables other software to run suitably, by interfacing with hardware and with other software.

Tuesday, July 24, 2007

Hunt

A hunt is a doings during which humans or animals chase some prey, such as wild or specially bred animals traditionally targeted species are known as game, in order to catch or kill them, either for food, sale or hobby, See hunting.

Wiktionary, the free dictionary, the word also applies collectively to a group of hunters practicing such a hunt, particularly with dogs. See fox hunting.

It may also, at least because circa 1200, take the form of a search for a specified target that can, unlike a hunting prey, remain unheard, as in specific compounds such as scavenger hunt or treasure hunt. Synonyms of hunt(ing) may have a similar resulting meaning, e.g. photo safari.

Monday, July 16, 2007

Robotics

It is the science and technology of robots, their plan, manufacture, and application.Robotics requires functioning information of electronics, mechanics, and software. A person functioning in the field is a roboticist. The word robotics was first used in issue by Isaac Asimov, in his science fiction short story "Runaround" (1941).

Although the outside and capabilities of robots vary extremely, all robots share the features of a mechanical, movable structure under some form of control. The chain is misshapen of links, actuators and joints which can allow one or more degrees of freedom. Most modern robots use open sequential chains in which each link connects the one before to the one after it. These robots are called serial robots and often look like the human arm. A few robots, such as the Stewart platform, use closed parallel kinematic chains. Other structures, such as those that imitate the mechanical structure of humans, diverse animals and insects, are relatively rare. However, the development and use of such structures in robots is a dynamic area of research. Robots used as manipulators have a finish effector mount on the last link. This end effector can be something from a welding mechanism to a mechanical provide used to manipulate the environment.

Wednesday, July 11, 2007

Software tools


Software tools for distributed processing include standard APIs such as MPI and PVM, and open source-based software solutions such as Beowulf and openMosix which make easy the creation of a supercomputer from a collection of ordinary workstations or servers. Technology like ZeroConf (Rendezvous/Bonjour) can be used to make ad hoc computer clusters to for specialized software such as Apple's shake compositing application. An easy programming language for supercomputers leftovers an open research topic in computer science.

Tuesday, June 26, 2007

Trawler

A Trawler is a type of recreational powerboat and is so named because it looks similar to a commercial fishing trawler. Like commercial fishing boat, a cruising trawler has a displacement hull and the pilothouse is raised. The engine and mechanical room is located under the main cabin and can be fairly large. Generally trawlers have a single engine and a large fuel capacity. Combined with the competence of a displacement hull, trawlers usually can go far before needing to be refueled and can be home for between 2 to 8 persons for many days and can be a permanent home.
Its ranges vary from 30 to 70 feet.

Tuesday, June 12, 2007

Raft

A raft is some flat floating structure for travel over water. It is the most basic of boat design, characterized by the lack of a hull. Instead, rafts are kept afloat using any mixture of buoyant materials such as wood, sealed barrels, or overstated air chambers. Traditional or antique rafts are constructing of wood or reeds. Present rafts may also use pontoons, drums, or extruded polystyrene blocks. Hot-air balloon rafts use durable, multi-layered rubberized fabrics, Depending on it’s utilize and size; it may contain a superstructure, masts, or rudders.
Timber rafting is used by the logging industry for the carrying of logs, by tying them together into rafts, and traveling or pulling them down a river. This method was very common up until the center of the 20th century but is now hardly used.
The type of raft used for spare time rafting is almost completely an inflatable boat, manufactured of flexible materials for use on whitewater.

Wednesday, June 06, 2007

Pirogue

A pirogue is a small, flat-botomed boat of a design associated mainly with West African fishermen and the Cajuns of the Louisiana marsh. These boats are not typically intended for over-night travel but are light and small sufficient to be easily taken onto land. The design also allows the pirogue to move through very shallow water and be simply turned over to drain any water that may get into the boat. The pirogue's motion comes from paddles that contain one blade (as opposed to a kayak paddle, which has two). It can also be punted with a pole in low water.
There is not one pirogue plan, are several. Besides small pirogues as seen on the picture, there are also pirogues that can hold up to ten men with paddles and also characteristic a main sail. These are not planned (and should not be used) for open waters. They are only (and best) used close to shore.

Monday, May 28, 2007

Junk

A junk is a Chinese marine vessel. The English name comes from Malay dugong or jong. Junks were initially developed during the Han Dynasty (220 BC-200 AD) and further evolved to symbolize one of the most successful ship types in history.
The organization and flexibility of junk sails make the junk easy to sail, and fast. Unlike a conventional square rigged ship the sails of a junk can be moved inward, toward the long axis of the ship, allow the junk to sail into the wind.
The sails include more than a few horizontal members ("battens") which provide shape and strength. -The sails can also be easily reefed and familiar for fullness, to accommodate various wind strengths. The battens also make the sails more resistant than traditional sails to large tears, as a tear is naturally limited to a single "panel" between battens. Junk sails have much in common with the most aerodynamically well-organized sails used today in windsurfers or catamarans, although their design can be traced back as early the 3rd century AD.
The standing chains are simple or absent. The sail-plan is also extending out between multiple masts, allowing for a powerful sail surface, and a good repartition of efforts, an innovation adopted in the West around 1304. [Citation needed] The rig allows for good marine into the wind.

Sunday, May 20, 2007

Hydrofoil

It is a boat with wing-like foils mounted on struts under the hull. As the craft put into its speed the hydrofoils make better enough lift for the boat to become foil borne - i.e. to raise the hull up and out of the water. This consequence has a great decrease in drag and a corresponding increase in speed.
Early hydrofoils worn U-shape foils. Hydrofoils of this type are recognized as surface-piercing since portions of the U-shape hydrofoils will rise over the water surface when foil borne. Modern hydrofoils make use of T-shape foils which are fully submerged. Fully submerged hydrofoils are fewer subjected to the effects of wave action, and are therefore steadier at sea and are more relaxed for the crew and passengers. This type of pattern, however, is not self-stabilizing. The angle of attack on the hydrofoils wants to be adjusted endlessly in accordance to the changing conditions, a control process that is performed by computers. Failure to make the proper adjustments will result in the foil borne hull dipping aggressively back into the sea.

Sunday, May 13, 2007

Fishing boat

A fishing boat can range from two-person enjoyment fishing boats up to 7-8 ton commercial fishers that can drag in over a billion fish at one time. Island nations like Japan rely on the fishing industry to give food. Fishing is also well-liked in places like the U.S, where it is often done for sport rather than for food. Fishing is extremely fixed in the American culture.
A lot of marinas and harbors cater to fishing boats and sport boats. It is an extensive array of business that make its living helpful for and service this boating and fishing community. Both leisure fishermen and commercial enterprises share the majority harbors.

Monday, April 02, 2007

Human-powered transport

Human-powered transport is transport of person(s) and or goods motorized by human muscle.
Like animal-powered transport, human-powered transport has been in continuation since time immemorial in the form of walking, running and swimming. However modern technology has led to machines to improve human-power. Although motorization has compact the effort in transport, many human-powered machines stay popular for leisure or exercise and for short distance travel. Human-powered transport is frequently the only (reliable) power source available in underdeveloped or inaccessible regions, and may be measured an ideal form of sustainable transportation.

Tuesday, March 20, 2007

Carriage

The standard definition of a carriage is a four-wheeled horse drawn personal passenger vehicle with leaf springs or leather robust for postponement, whether light, smart and fast or large and relaxed. Compare the public conveyance stagecoach, charabanc, and omnibus.
A medium that is not sprung is a wagon. An American buckboard or Conestoga wagon or "prairie schooner" was never taken for a carriage, but a waggonette was an enjoyment vehicle, with lengthways seats.
The word car meaning "wheeled vehicle", came from Norman French at the start of the 14th century; it was absolute to cover automobile in 1896.
In the British Isles and many Commonwealth countries, a railway carriage (also called a coach) is a railroad car planned and prepared for transporting passengers.
In the United States, a baby carriage is a wheeled transportation for recline infants (in English outside North America: perambulator or pram), often with a hood that can be adjusted to protect the baby from the sun.

Sunday, January 21, 2007

Lotion

A lotion is a low- to medium-viscosity medicated or non-medicated topical preparation intended for application to unbroken skin. Most lotions are oil-in-water emulsions but water-in-oil lotions are also formulated. Lotions are usually applied to external skin with a clean cloth, cotton wool or gauze. The key components of a lotion emulsion are the aqueous and oily phases, an emulgent to prevent separation of these two phases, and, if used, the drug substance or substances. A wide variety of other ingredients such as fragrances, glycerol, dyes, preservatives, vitamins, proteins and stabilizing agents are commonly added to lotions.

It is not uncommon for the same drug ingredient to be formulated into a lotion, cream and ointment. Creams are the most convenient of the three but are inappropriate for application to regions of hairy skin such as the scalp, while a lotion is less viscous and may be readily applied to these areas. Lotions also have an advantage in that they may be spread thinly compared to a cream or ointment and may economically cover a large area of skin. Non-comedogenic lotions are recommended for use on acne prone skin.

Monday, January 08, 2007

Unusual Twinnings

There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case. Among fraternal twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse, either within one menstrual cycle or, even more rarely, later on in the pregnancy. This can lead to the possibility of a woman carrying fraternal twins with different fathers. This phenomenon is known as heteropaternal super fecundation. One 1992 study estimates that the frequency of heteropaternal super fecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.

Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes. The probability of this is so vanishingly small that multiples having different genders are universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different genders it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.

Monday, January 01, 2007

Biodiversity

Biodiversity or biological diversity is the diversity of life. There are a number of definitions and a measure of biodiversity. Biodiversity has no single standard definition. The most basic definition is "variation of life at all level of biological organization". Another definition holds that biodiversity is a measure of the relative diversity among organisms present in diverse ecosystems. "Diversity" in this definition includes diversity within a species and among species, and comparative diversity among ecosystems.

A third definition that is often used by ecologists is the "totality of genes, species, and ecosystems of a region". An advantage of this description is that it seems to describe most circumstances and present a unified view of the traditional three levels at which biodiversity has been identified.